How can you use vector blur in Cycles with fluids?
In short:
Cycles (as of Blender 2.67) does not support fluids blur yet.
A GSOC 2013 project might bring that feature soon. 



For now, as a work around, you can try:


Render your primary pass in cycles. Save to EXR.
Render the same scene in Blender Internal, save the Z and Vector pass to another EXR. 
Combine the two set of EXRs with Nodes with a vector blur node. Using the Color input from the Cycles EXR, and use the Z and Vector input from the BI EXR.